Controlling the Curie temperature in (Ga,Mn)As through location of the Fermi level within the impurity band

The ferromagnetic semiconductor (Ga,Mn)As has emerged as the most studied material for prototype applications in semiconductor spintronics. Because ferromagnetism in (Ga,Mn)As is hole-mediated, the nature of the hole states has direct and crucial bearing on its Curie temperature T(C). It is vigorously debated, however, whether holes in (Ga,Mn)As reside in the valence band or in an impurity band. Here we combine results of channelling experiments, which measure the concentrations both of Mn ions and of holes relevant to the ferromagnetic order, with magnetization, transport, and magneto-optical data to address this issue. Taken together, these measurements provide strong evidence that it is the location of the Fermi level within the impurity band that determines T(C) through determining the degree of hole localization. This finding differs drastically from the often accepted view that T(C) is controlled by valence band holes, thus opening new avenues for achieving higher values of T(C).     

Impurity band in in-doped CdCr2Se4 observed in the spectral dependence of the photoconductance

The spectral dependence of the photoconductance of In-doped CdCr₂Se₄ crystals was measured in the photon energy range from 0.2 eV to 2.5 eV. At a temperature of 80 K, i.e. below the Curie temperature of 132 K, two peaks of the impurity photoconductivity are observed. The peak energy of 0.38 eV agrees with the thermal activation energy determined from the slope of the ? vs T curves at higher temperatures. This agreement is an experimental evidence for the impurity band formed by the In donors and the Se vacancy donors which should be responsible for the large negative magnetoresistance due to “magnetic impurity states” in these crystals. At a temperature of 155 K, i.e. above the Curie temperature, the structure of the photoconductance vanishes due to the thermal activation of electrons from occupied donor levels to the conduction band.     

Curie Temperatures of III–V Diluted Magnetic Semiconductors Calculated from First-Principles in Mean Field Approximation

Electronic structure and ferromagnetism in III–V compound-based diluted magnetic semiconductors (DMS) are investigated based on first-principles calculations by using the Korringa-Kohn-Rostoker method combined with the coherent-potential-approximation. The stability of the ferromagnetic phase in GaN-, GaAs-, GaP-, GaSb-based DMS is investigated systematically. The calculations show that 3d-impurities from the first-half of the transition metal series favor the ferromagnetic state, while impurities from the latter-half of the series exhibit spin-glass behavior. This chemical trend in the magnetism is explained by the double exchange mechanism taking the local symmetry at the impurity gap states into account. Curie temperatures of GaAs- and GaN-based DMS are estimated by using the Heisenberg model in a mean field approximation with the parameters calculated from first-principles. It is suggested that room-temperature ferromagnetism can be realized in these systems.     

Optical and Magnetic Properties of Ten-Period InGaMnAs/GaAs Quantum Wells

Ten layers of InGaMnAs/GaAs multiquantum wells (MQWs) structure were grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of the ferromagnetic structure was confirmed in the InGaMnAs/GaAs MQWs structure, and have ferromagnetic ordering with a Curie temperature, T _C=50 K. It is likely that the ferromagnetic exchange coupling of the sample with T _C=50 K is hole-mediated resulting in Mn substituting In or Ga sites. PL emission spectra of the InGaMnAs MQWs sample grown at a temperature of 170 °C show that an activation energy of the Mn ion on the first quantum confinement level in InGaAs QW is 32 meV and impurity Mn is partly ionized. The fact that the activation energy of 32 meV of Mn ion in the QW is lower than an activation energy of 110 meV for a substitutional Mn impurity in GaAs, indicating an impurity band existing in the bandgap due to substitutional Mn ions.     

The structural stability and magnetooptical properties of the NiAs phase of the manganese-antimony system

The ferromagnetic properties of manganese antimony (MnSb), in contrast to the corresponding compounds of arsenic and bismuth, do not show instabilities associated with first-order phase transitions near the magnetic ordering temperature. However, the magnetic properties of the manganese-antimony system are sensitive to the presence of interstitial cations. For example, Mn1+xSb phases have been reported (0 < x < 0.2) with Curie temperatures varying between 300 and 90°C. Although MnSb is free from first-order phase changes, bulk samples of Mn1.2Sb were found to be magnetically unstable on thermal cycling through their Curie temperature. Repeated thermal cycling resulted in a shift of the Curie temperature from 90 to ∼300°C ; a value typical of stoichiometric MnSb. Lattice constant and magnetization measurements are consistent with this magnetic aging being associated with the migration of manganese ions away from the bipyramidal interstices in which they are known to couple antiferromagnetically with the octahedral lattice site manganese ions. Polycrystalline films of stoichiometric MnSb were prepared by RF sputtering and were shown to have optical absorption and Faraday rotation comparable to isostructural MnBi. Attempts to prepare films of Mn1.2Sb with a Curie temperature of 90°C always resulted in material with the magnetic properties of stoichiometric MnSb, and lattice constant data strongly suggest that we were unable to populate the interstitial cation sites in the films.     

The Dilute Charged Impurity Effects on Electronic Heat Capacity and Magnetic Susceptibility of Ferromagnetic Silicene Sheet

Starting from the Kane-Mele Hamiltonian, Dirac cone approximation and self-consistent Born approximation (SCBA), the effects of dilute charged impurity doping on electronic heat capacity (EHC) and magnetic susceptibility (MS) of a two-dimensional material ferromagnetic graphene’s silicon analog, silicene, are investigated within the Green’s function approach that allows the accurate assessment of dynamic of carriers. Also, we have studied the behavior of these quantities in the presence of applied external electric field (AEEF). Our results show that the inversion symmetry is broken by impurity doping. According to EHC behaviors, the band gap is decreased with impurity concentration (IC), impurity scattering strenght (ISS), and AEEF. As a remarkable point, a phase transition from ferromagnetic to paramagnetic and antiferromagnetic has been observed. On the other hand, there is a critical impurity concentration for maximum response to the temperature and magnetic field. The effect of AEEF leads to the decrease of band gap and phase transition depends on the direction of AEEF.     

Potential applications of magnetic rare earth compunds

Chemistry and physics of the rare earth elements and their compounds are in a rapidly expanding state of development. Many new materials with interesting magnetic properties have recently become available. The potential of lanthanon compounds as magnetic materials in electrical engineering is evaluated by comparing the basic properties of lanthanons and transition metals. The lanthanides show certain unique properties resulting from the localization of their magnetic4flevels. Unfortunately, their Curie temperatures are relatively low. The probability of finding ferromagnetic lanthanide compounds with high Curie temperatures is explored. A survey of magnetic properties of metallic and nonmetallic lanthanide materials indicates possible applications for which they can compete favorably with other materials.     

Effect of Phosphorus Substitution on Stability,Electronic, and Magnetic Properties of SiC Hybrid

The effect of different concentrations of P-atom-substituted SiC sheet were studied using ab initio calculations within the density functional theory. For dosage concentrations ranging from 1/32 to 8/32, structural, electronic, and magnetic properties are reported and discussed. Thermal stability performed using phonon dispersion analysis and molecular dynamic calculations shows that P-substituting SiC hybrid is an exothermic process. All the substituted systems are indirect band gap semiconductors with strongly bonded C-, Si-, and P-atoms accompanied by an appreciable electron transfer from Si- and P- towards C-atoms. Doping with P decreases the gap of pure SiC and varies it in the interval (1.37–1.81 eV) for different dosages. Moreover, spin-polarized calculations show that the configurations with 1/32 to 4/32 concentrations exhibit a magnetic behavior and Curie temperature is determined for ferromagnetic structures. For dosage concentrations larger than 5/32, the structures are nonmagnetic.     

The Effect of Dilute Charged Impurity on the Electronic Heat Capacity and Magnetic Susceptibility of Ferromagnetic MoS<Subscript>2</Subscript>

In this paper, the effects of dilute charged impurity doping on electronic heat capacity (EHC) and magnetic susceptibility (MS) of a two-dimensional material ferromagnetic gapped graphene-like, MoS₂, are investigated within the Green’s function approach by using the Kane-Mele Hamiltonian and self-consistent Born approximation (SCBA) at Dirac points. Our findings show that there is a critical impurity concentration (IC) and scattering strength (ISS) for each valley in EHC and MS curves. Also, we have found that the spin band gap decreases with impurity only for valley K, ↑ and \(K^{\prime }, \downarrow \) due to the existence of inversion symmetry between valleys. On the other hand, a magnetic phase transition from ferromagnetic to antiferromagnetic and paramagnetic has been observed. The increase of scattering rate of carriers in the presence of impurity is the main reason of these behaviors.     

Search for thermometers with low magnetoresistive effects: platinum-cobalt alloy

Measurements performed on thermometers made of dilute ferromagnetic alloys of Pt with 0.45, 0.50, 0.75, 1.06 and 2.15 at% Co, in the temperature range 2–28 K and in magnetic fields up to 6 T showed a large region of negative magnetoresistivity that limits the magnetic error at low temperatures and field values. With 0.5 at% Co or less, the change of the magnetic error with T and B was smooth, while a magnetic transition has been observed with higher cobalt concentrations. Curie temperature has been found to occur at 6.0 K, 14.2 K and, by extrapolation, 32 K for alloys with 0.75, 1.06, and 2.15 at%Co respectively. The transition resulted in a second order discontinuity of the R-T characteristics at B = 0 and B = 3.8 ± 0.2 T with 0.75 at%Co and 8.0 ± 0.5 T with 1.06 at%Co, while for other field values the discontinuity was first-order. Hence, this alloy can be used for thermometry only with very low cobalt concentrations: with 0.3 at%Co it would be possible to limit the magnetic error within ±0.2 K up to 2 T and done to 5 K.     

Proprietes electriques et magnetiques de Mn0,25NbS2 et Mn0,33NbS2

Crystals of Mn_0.25NbS₂ and Mn_0.33NbS₂ phases were grown by iodine vapor transport. Their accurate composition has been determined and single phase specimens used for studies of magnetic and electrical properties. Both compounds show ferromagnetic interactions at low temperature. Curie paramagnetic temperatures are 120°K for Mn_0.25NbS₂ and 53°K for Mn_0.33NbS₂. The experimental magnetic moments indicate an intermediate valence state (between +2 and +3) for manganese in the compounds. Resistivity and Hall measurements show a metallic character and coincide with the intermediate valence state. Interpretations are given in terms of a simple band model, and an analysis of the magnetic interactions is proposed.     

Defect mediated magnetic interaction and high Tc ferromagnetism in Co doped ZnO nanoparticles

Structural, optical and magnetic studies have been carried out for the Co-doped ZnO nanoparticles (NPs). ZnO NPs are doped with 3% and 5% Co using ball milling and ferromagnetism (FM) is studied at room temperature and above. A high Curie temperature (Tc) has been observed from the Co doped ZnO NPs. X-ray diffraction and high resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption and photoluminescence studies on the doped samples show change in band structure and oxygen vacancy defects, respectively. Micro-Raman studies of doped samples shows defect related additional strong bands at 547 and 574 cm(-1) confirming the presence of oxygen vacancy defects in ZnO lattice. The field dependence of magnetization (M-H curve) measured at room temperature exhibits the clear M-H loop with saturation magnetization and coercive field of the order of 4-6 emu/g and 260 G, respectively. Temperature dependence of magnetization measurement shows sharp ferromagnetic to paramagnetic transition with a high Tc = 791 K for 3% Co doped ZnO NPs. Ferromagnetic ordering is interpreted in terms of overlapping of polarons mediated through oxygen vacancy defects based on the bound magnetic polaron (BMP) model. We show that the observed FM data fits well with the BMP model involving localised carriers and magnetic cations.     

Electrical control of the ferromagnetic phase transition in cobalt at room temperature

Electrical control of magnetic properties is crucial for device applications in the field of spintronics. Although the magnetic coercivity or anisotropy has been successfully controlled electrically in metals as well as in semiconductors, the electrical control of Curie temperature has been realized only in semiconductors at low temperature. Here, we demonstrate the room-temperature electrical control of the ferromagnetic phase transition in cobalt, one of the most representative transition-metal ferromagnets. Solid-state field effect devices consisting of a ultrathin cobalt film covered by a dielectric layer and a gate electrode were fabricated. We prove that the Curie temperature of cobalt can be changed by up to 12 K by applying a gate electric field of about ±2 MV cm(-1). The two-dimensionality of the cobalt film may be relevant to our observations. The demonstrated electric field effect in the ferromagnetic metal at room temperature is a significant step towards realizing future low-power magnetic applications.     

Sur les proprietes magnetiques du fluorure KxVF3β (0,45 ⩽ x ⩽ 0,55)

The magnetic properties of β-K_xVF₃ (0,45 ≤ x ≤ 0,55), have been investigated.At high temperature (T > 150 K), the molar susceptibility follows a Curie-Weiss law. The experimental values of the Curie constant and of the effective moment are in good agreement with the calculated values. The paramagnetic Curie temperatures are negative and involve preponderant antiferromagnetic interactions. At low temperature a strong ferromagnetic component appears (T < 58 K); its origin is discussed and several hypotheses are proposed.     

Measurement of an Iso‐Curie Temperature Line of a Co?Cr?Mo Solid Solution by Magnetic Force Microscopy Imaging on a Diffusion Multiple

The 24 °C iso‐Curie temperature line of a Co? Cr? Mo fcc solid solution is obtained by performing magnetic force microscopy (MFM) imaging on solid solution compositions created in a diffusion multiple. The MFM imaging clearly reveals the boundary that separates the paramagnetic region without magnetic domains from the ferromagnetic region with domains. Compositional analysis along the boundary yields a constant Curie temperature (24 °C) composition line. Such a measurement is more efficient than one‐alloy‐at‐a‐time tests and can be used to screen new ferromagnetic materials.     

Ferromagnetism of ZnO and GaN: A Review

The observation of ferromagnetism in magnetic ion doped II–VI diluted magnetic semiconductors (DMSs) and oxides, and later in (Ga,Mn)As materials has inspired a great deal of research interest in a field dubbed “spintronics” of late, which could pave the way to exploit spin in addition to charge in semiconductor devices. The main challenge for practical application of the DMS materials is the attainment of a Curie temperature at or preferably above room temperature to be compatible with junction temperatures. Among the studies of transition-metal doped conventional III–V and II–VI semiconductors, transition-metal-doped ZnO and GaN became the most extensively studied topical materials since the prediction by Dietl et al., based on mean field theory, as promising candidates to realize a diluted magnetic material with Curie temperature above room temperature. The underlying assumptions, however, such as transition metal concentrations in excess of 5% and hole concentrations of about 10²⁰ cm⁻³, have not gotten as much attention. The particular predictions are predicated on the assumption that hole mediated exchange interaction is responsible for magnetic ordering. Among the additional advantages of ZnO-and GaN-based DMSs are that they can be readily incorporated in the existing semiconductor heterostructure systems, where a number of optical and electronic devices have been realized, thus allowing the exploration of the underlying physics and applications based on previously unavailable combinations of quantum structures and magnetism in semiconductors. This review focuses primarily on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs. One of the desirable outcomes is to obtain carrier mediated magnetism, so that the magnetic properties can be manipulated by charge control, for example through external electrical voltage. We shall first describe the basic theories forwarded for the mechanisms producing ferromagnetic behavior in DMS materials, and then review the theoretical results dealing with ZnO and GaN. The rest of the review is devoted to the structural, optical, and magnetic properties of ZnO- and GaN-based DMS materials reported in the literature. A critical review of the question concerning the origin of ferromagnetism in diluted magnetic semiconductors is given. In a similar vein, limitations and problems for identifying novel ferromagnetic DMS are briefly discussed, followed by challenges and a few examples of potential devices.     

Origin of Magnetism from Native Point Defects in ZnO

Based on the first-principle calculations by using the Korringa?CKohn?CRostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA), we study theoretically the electronic and magnetic properties of different point defects in ZnO, which are Zinc interstitials (Zn_i), Zinc antisites (Zn_O), Oxygen interstitials (O_i) and Oxygen antisites (O_Zn) defects in ZnO. The supercell calculations were also performed using the full potential local-orbital (FPLO) band structure scheme. This work presents detailed information about total and local density of states at some concentrations of these defects; the stability of the ferromagnetic state compared with the spin-glass state is investigated by comparing calculating their total energy. The results show on one hand that Zn_i and Zn_O produce a shallow donor bellow the bottom of the conduction band (CB), while O_i and O_Zn produces the shallow acceptors above the top of the valence band (VB), and moment magnetic; on other hand that the ferromagnetic state is more stable than the spin-glass in Oxygen interstitials (O_i) and vice versa for oxygen antisites (O_Zn) of native point defects in ZnO. The other native point defects (Zn_i, Zn_O, V_O, and V_Zn) have a zero magnetic moment. The results show that the Curie temperature increases with the concentration of interstitial oxygen.     

Composes ferrimagnetiques fluores de structure Na2SiF6

The magnetic properties of NaMnCrF₆ and of its antitype LiMnCrF₆, both related to trigonal Na₂SiF₆ structure, have been investigated. They are ferrimagnetic with respective Curie temperatures T_C of 21 and 23° K. The values of their asymptotic Curie temperatures θ_P involve ferromagnetic interactions. The observed difference between their respective saturation magnetization (6 and 4 μB/molecule) is discussed and related to the different types of cationic order, using an approached magnetic model with spins oriented along [001] axis. Magnetic properties of some substituted phases are also described.     

On the optimization of soft-magnetic properties of metallic glassesby dynamic current annealing

A dynamic relaxation process has been developed that allows optimization of the soft-magnetic properties of ferromagnetic amorphous tapes during the winding process using Joule heating. Best magnetic properties of commercial FeSiB amorphous tapes are attained after only about ten seconds of annealing in air far below the crystallization temperatures but above the Curie temperature. It is found that structural and stress relaxation phenomena follow nearly the same temperature dependences in this annealing range     

Room Temperature Ferromagnetism in III–V-Based Diluted Magnetic Semiconductor GaCrN Grown by ECR Molecular-Beam Epitaxy

A new III–V nitride-based diluted magnetic semiconductor GaCrN has been successfully synthesized for the first time. X-ray diffraction measurement showed no existence of secondary phase in the GaCrN layers. They showed a ferromagnetic behavior with the Curie temperature of higher than 400 K, and clear saturation and hysteresis were observed in the magnetization versus magnetic field curves at all measuring temperatures (10–400 K). Specially, GaCrN magnetization data show no paramagnetic component at low temperature, which are superior characteristics over GaMnN. Photoluminescence emission from GaCrN layer was observed at around 3.29 eV at 10–300 K.